1. Field of the Invention
The present invention relates to a ceramic superconductor wire used as an electric wire, a cable, a coil winding, an electromagnetic shielding body, and a current lead wire for supplying a current to a superconductor element and a method of manufacturing the ceramic superconductor wire and, more particularly, a ceramic superconductor wire having a small AC current loss and a method of manufacturing the ceramic superconductor wire.
2. Description of the Related Art
In recent years, various types of ceramic superconductors represented by chemical formulas such as YBa.sub.2 Cu.sub.3 O.sub.7-y, Bi.sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.x, Tl.sub.m Ba.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.x (m=1, 2; n=1, 2, 3, 4, 5, . . .), and (Nd.sub.1-x Ce.sub.x).sub.2 CuO.sub.4-y have been developed. Critical temperatures (Tc) of these ceramic superconductors reach 125 K.
Since these ceramic superconductors have poor workability, they are used in a state wherein a high-conductivity metal layer 2 is formed to cover a ceramic superconductor layer 1, as shown in FIG. 11. The form of such a ceramic superconductor is obtained by filling a ceramic superconductor or its precursor powder in a metal pipe or the like, rolling the pipe, and performing a predetermined heat treatment of the rolled pipe.
The high-conductivity metal layer consisting of a ceramic superconductor serves as a heat sink against heating caused by a flux jump as a rapid shift of a magnetic flux in the ceramic superconductor layer during energization. The high-conductivity metal layer also serves as a current bypath. A high-conductivity metal such as Ag or Cu having high thermal and electric conductivities is used as the above high-conductivity metal layer. In addition to these functions, a high-conductivity metal layer such as an Ag or Cu layer protects ceramic superconductors from external harmful substances and mechanically reinforces the ceramic superconductor.
A conventional ceramic superconductor wire covered with a high-conductivity metal layer such as an Ag or Cu layer causes the metal layer to generate an eddy current upon AC current energization of the ceramic superconductor wire.
The following problem is also posed when the above superconductor is used for a current lead. An SMES power generator, an MHD power generator, a fusion reactor, a linear motor car, a medical MRI, an accelerator magnet, and the like have been developed as low-temperature equipment using superconductor magnets. A current is supplied from an external power source to such low-temperature equipment through a current lead wire. The current lead wire consists of a metal material such as Cu, Cu-Ag, or Cu-P. Metal conductors are subjected to Joule heating upon current energization and absorption of a large amount of external heat. A large amount of coolant is undesirably evaporated.
There has been proposed use of a metal or intermetallic superconductor such as Nb-Ti and Nb.sub.3 Sn as the above current lead wire which does not produce Joule heat by cooling the metal or intermetallic superconductor to a liquid He temperature (4.2 K) to nullify an electric resistance. Such a superconductor cannot be used near any equipment except for low-temperature equipment cooled to the liquid He temperature and cannot sufficiently enhance its effect.
In most recent years, ceramic superconductors such as YBa.sub.2 Cu.sub.3 O.sub.7, Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8, Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10, Tl.sub.2 Ba.sub.2 CaCu.sub.2 O.sub.8, Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 having critical temperatures (Tc) as high as 8 to 125 K, which allow substances to be superconductive at the liquid N.sub.2 temperature, have been found.
These ceramic superconductors have high critical temperatures (Tc). When these superconductors are used as current lead wires, a liquid He cooling effect from the low-temperature equipment can be sufficiently utilized. Ceramic superconductors have low heat conductivities unlike metal materials such as Cu, and have an advantage in low absorption of external heat. Therefore, these ceramic superconductors have been expected as practical current lead wires.
Since a ceramic superconductor is, however, brittle, the following process is required. For example, a ceramic superconductor powder is filled in a metal tube as of Ag or Au, and the metal tube is then extruded, pressed, rolled, drawn or swaged. In this case, the metal tube must have good workability and a high purity, and have a ratio of a wall thickness to a sectional area to be 30 to 50% or more because the ceramic superconductor is hard. For this reason, a large amount of heat is conducted to the low-temperature equipment from a metal layer of a current lead wire rolled from the metal tube. A large amount of liquid He as a coolant is consumed, thus resulting in poor economy.